Mediators are produced by human macrophages and lymphocytes that have a number of in vitro immunological effects. Macrophages can be activated by a wide variety of agents including phorbol myristic acetate, muranyl dipeptide, "transfer factor", activated lymphocytes and lymphokines to produce a lymphocyte activating factor (LAF). LAF activity elutes in two peaks off BioGel P100 columns at 60,000 and 14,000-20,000 daltons. This LAF containing high MW peak augments immunological reactions by 1) stimulating or enhancing the proliferation of mouse thymocytes or human peripheral T cells, 2) increasing the expression of Ly-1 antigen on mouse thymocytes, 3) activating human lymphocytes to produce another lymphokine and 4) others have shown that LAF increases helper cell promotion of antibody responses. In contrast, antigens can induce lymphocytes to produce a factor (LMF) which activates human B cell proliferation. This factor over-laps the lower MW LAF peak. Thus, LAF and LMF can amplify immunological reactions by augmenting lymphocyte proliferation, differentiation and thus promoting lymphocyte functions.